The present invention relates to an apparatus for watching around a vehicle by photographing surrounding scenes at a driver""s dead angle in such a condition that does not offer unobstructed view to a driver ahead or behind the vehicle and on the right or left side of the vehicle so as to display the scenes to the driver.
FIG. 13 is a block diagram of a conventional apparatus for watching around a vehicle. This apparatus for watching around a vehicle 1 comprises an imaging device 11 disposed on the outer side of the vehicle and used for picking up scenes in the two opposed directions around the vehicle, an image inversion processing portion 12 for subjecting image signals supplied from the imaging device 11 to image inversion processing, and a display portion 15 that is set inside the vehicle chamber and used for displaying the image signals obtained from the image inversion processing portion 12.
The imaging device 11 has a shade casing 3 provided with a pair of right and left transmission window portions 2R and 2L formed of transparent glass in either side of the casing 3, which contains a prism 4 in the form of an isosceles triangle in section with its vertical angle 5 directed to the front part of the casing (the upper part of the casing 3 in FIG. 13), the right and left sides 8R and 8L of the prism 4 corresponding to the equal sides of the isosceles triangle being directed to the respective right and left transmission window portions 2R and 2L, and an imaging element 10 disposed to the rear of the prism 4 and used for converting rays of light 18R and 18L into image signals, the rays of light being incident on the respective sides 8R and 8L of the prism 4 and subjected to an optical path change within the prism 4 and guided via an imaging lens 9 onto an imaging plane.
The imaging device 11 is fitted to a front bumper, for example, so that its front part (the front part of the casing 3) is directed ahead and that the transmission window portions 2R and 2L are respectively directed right and left. In this condition, the ray of light 18L corresponding to the left side scene is as shown in FIG. 13 passed through the transmission window portion 2L and the side 8L of the prism 4, directly incident on the side 8R of the prism 4, reflected from the interior of the side 8R of the prism 4, emitted from the rear side 8B of the prism 4 and formed into an image through the imaging lens 9, which image is guided to the left half 10L of the imaging plane of the imaging element 10. In consideration of the ray of light 18R corresponding to the right side scene from the standpoint of bilateral symmetry, the ray of light is guided to the right half 10R of the imaging plane of the imaging element 10 likewise. Then both rays of light are converted by the imaging element 10 into image signals.
The right and left side scenes are picked up and the signals of the images thus picked up are subjected to the image inversion processing in the image inversion processing portion 12 before being supplied to the display portion 15 as shown in FIG. 13. In the display portion 15, the left side scene taken in from the transmission window portion 2L is displayed as the left half of the image on the left half 15L of a screen, whereas the right side scene taken in from the transmission window portion 2R is displayed as the right half of the image on the right half 15R of the screen.
In an apparatus 1 for watching around a vehicle, however, when a ray of light 21a emitted from a headlamp 21 is passed through the transmission window portion 2R and the side 8R of the prism 4 and directly incident on the rear side 8B of the prism 4 and totally reflected from the rear side 8B of the prism 4 while an oncoming vehicle with its headlamp 21 turned on is approaching, for example, the ray of light 21a is emitted to the side 8L of the prism 4 and incident on the transmission window portion 2L. At this time, though most of the ray of light 21a (96.1%) incident on the transmission window portion 2L is passed through the transmission window portion 2L and emitted outside, the remaining ray of light (3.2%) is incident on the side 8L of the prism 4 as reflected light 21b. 
The reflected light 21b is passed through the side 8L of the prism 4, directly incident on the side 8R of the prism 4, reflected from the interior of the side 8R of the prism 4, emitted from the rear side 8B of the prism 4 and guided via the imaging lens 9 to the left half 10L of the imaging plane of the imaging element 10. Consequently, the reflected light 21b is subjected to image processing on the left half 10L of the imaging plane of the imaging element 10 after being synthesized with the ray of light 18L corresponding to the left side scene.
In this case, an image 21B deriving from the light 21b reflected from the transmission window portion 2L with the ray of light 21a emitted from the headlamp 21 and taken in from the transmission window portion 2R being passed therethrough is superposed on an image 22L of the left side scene taken in from the transmission window portion 2L on the left half 15L of the screen whereby to indicate an error.
In this manner, in the conventional apparatus 1 for watching around a vehicle, part of the scene taken in from one side of the transmission window portions 2R and 2L falls on the other side of the screens 15R and 15L of the display portion 15, thus causing an error in displaying, and the problem is that the visibility of a driver is deteriorated.
 less than First Proposal as an Example greater than 
Therefore, the present applicant has made a first proposal by ways of example as shown in FIG. 14 (the Japanese Patent Application No. 2000-139754). Under the first proposal, in order to prevent part of the scene taken in from the left transmission window portion 2L from falling on the right half 15R of the screen in the display portion 15 and thus causing an error in displaying or to prevent part of the scene taken in from the right transmission window portion 2R from falling on the left half 15L of the screen therein and thus causing an error in displaying likewise, the angles of the transmission window portions 2R and 2L with the rear side 8B of the prism 4 have properly been set. Consequently, even though there is produced reflected light 21b that may be incident on the imaging element 10 as light is reflected from the interiors of the transmission window portions 2R and 2L, the reflected light 21b can be prevented from being emitted from the rear side 8B of the prism 4 after being totally reflected from the rear side 8B of the prism 4.
Since any unnecessary ray of light is prevented from being emitted from the rear side 8B of the prism 4, the ray of light can be prevented from falling on the screens 15R and 15L of the display portion 15. The visibility of the driver can also be improved accordingly.
 less than Second Proposal as an Example greater than 
Further, the present applicant has also made a second proposal by ways of example as shown in FIG. 15 (the Japanese Patent Application No. 2000-231289). Under the second proposal, a shading portion 3a for cutting off light substantially laterally crossing a field in front of the prism 4 is provided between the vertical angle portion 5 of the prism 4 and the front end wall 28 of the casing 3A. With this arrangement, even though an ray of light from outside is incident ahead of the vertical angle portion 5 of the prism 4 so as to cross the field in front of the prism 4, the light is cut off by the shading portion 3a, so that a phantom image is made preventable from falling on the screen as a result of the fact that the light crossing the field in front of the prism 4 is reflected from the interiors of the transmission window portions 2R and 2L and enters the prism 4.
Under the second proposal, the prism 4 is fixedly held by a prism securing member 32 and disposed within the casing 3A. The prism securing member 32 is used to fix the prism 4 by, for example, clamping right and left corner portions in the rear portion of the prism 4 from both right and left sides and also serves to cut off any unnecessary ray of light from being incident upon the rear areas on the sides 8R and 8L of the prism 4.
Even in the second proposal, as such an unnecessary ray of light is prevented from entering the prism 4, the ray of light is prevented from falling on the screens 15R and 15L of the display portion 15 whereby to improve the visibility of the driver.
 less than New Problem greater than 
In the case of the first and second proposals, though the unnecessary ray of light can be prevented from entering the imaging element 10 to a certain extent, the present applicant has further discovered a stray light path led to the imaging element 10 on the basis of detailed verification. FIG. 16 shows a path for stray light that is unpreventable according to the first and second proposals.
As shown in FIG. 16, assuming that a ray of light is emitted from P1 of the headlamp 21 of an oncoming vehicle, for example, in this case, the ray of light 21a emitted from the headlamp 21 is passed through the right transmission window portion 2R and then enters the prism 4 via a point P2 on the right side 8R of the prism 4. Further, the ray of light traveling in the prism 4 is totally reflected at a point P3 on the rear side 8B of the prism 4 and directed to the left side 8L of the prism 4. When the reflected light hits against a point P4 on the side 8L of the prism 4, about 4% of the reflected light is reflected thereat, directed to the rear side 8B of the prism 4 again, totally reflected at a point P5 and directed to the right side 8R of the prism 4. Further, about 4% of the reflected light is reflected at a point P6 and directed to the rear side 8B of the prism 4 again. Next, the reflected light is totally reflected at a point 7 on the rear side 8B of the prism 4 and directed to the side 8L of the prism 4. Then about 4% of the reflected light is reflected at a point P8 on the side 8L of the prism 4, totally reflected at a point P9 on the right side 8R of the prism 4 and moves out at a point P 10 on the rear side 8B of the prism 4. The ray of light thus slipped out causes stray light to fall on the display portion 15.
More specifically, the three rounds of total reflections from the rear side 8B of the prism 4 as shown in FIG. 16 may be said to ultimately and mainly cause the stray light to fall on the display portion 15. If, therefore, even one round out of the three rounds of total reflections is made avoidable, the ultimate degree of causing such stray light to fall on the display portion 15 is considered to be drastically reducible.
An object of the present invention is to provide an apparatus for watching around a vehicle so as to prevent an error in displaying as a result of the fact that stray light heretofore remain unpreventable by the first and second proposals made for reference falls on the other side of a screen in a display portion.
In order to solve the foregoing problems, an apparatus for watching around a vehicle as the invention described in aspect 1 comprises an imaging device including a casing disposed on the outer side of the vehicle and having a pair of right and left transmission window portions in either side of the casing, which casing contains a prism in the form of an isosceles triangle in section with its vertical angle directed to the front part of the casing, the right and left sides of the prism which correspond to the equal sides of the isosceles triangle being directed to the respective right and left transmission window portions, and an imaging element disposed to the rear of the prism and used for converting a ray of light into an image signal by concentrating light on an imaging plane via an imaging lens, the ray of light being incident on one of the right and left sides of the prism, reflected from the other side of the prism and emitted from the rear side of the prism, internal reflection preventive means for preventing stray light from undergoing total reflection in the prism being formed outside an effective area with respect to the imaging lens disposed to the rear side of the prism.
In the invention described in aspect 2, the internal reflection preventive means is an internal reflection preventive film having a refractive index greater by 0.766 time than the refractive index of the prism.
In the invention described in aspect 3, the internal reflection preventive means is an internal reflection preventive film having a refractive index greater by 0.9 time than the refractive index of the prism.
In the invention described in aspect 4, the internal reflection preventive film is formed of black paint which absorbs the stray light.
In the invention described in aspect 5, the internal reflection preventive means is an adhesive member for sticking to the rear side of the prism a buffer member for buffering the prism with respect to a predetermined holder for supporting the prism.
In the invention described in aspect 6, the refractive index of the adhesive member is greater by 0.766 time than the refractive index of the prism.
In the invention as described in aspect 7, the refractive index of the adhesive member is greater by 0.9 time than the refractive index of the prism.
In the invention as described in aspect 8, the adhesive member is colored black so as to absorb the stray light.
In the invention as described in aspect 9, the internal reflection preventive means includes a light scattering plane formed outside an effective area with respect to the imaging lens disposed to the rear of the prism.
In the invention as described in aspect 10, black paint for absorbing stray light is applied to the light scattering plane.
In the invention as described in aspect 11, light absorbing means for absorbing light resulting from the partial reflection of the stray light from the side of the prism is formed in part of the side of the prism.
An apparatus for watching around a vehicle as the invention described in aspect 12 comprises an imaging device including a casing disposed on the outer side of the vehicle and having a pair of right and left transmission window portions in either side of the casing, which casing contains a prism in the form of an isosceles triangle in section with its vertical angle directed to the front part of the casing, the right and left sides of the prism which correspond to the equal sides of the isosceles triangle being directed to the respective right and left transmission window portions, and an imaging element disposed to the rear of the prism and used for converting a ray of light into an image signal by concentrating light on an imaging plane via an imaging lens, the ray of light being incident on one of the right and left sides of the prism, reflected from the other side of the prism and emitted from the rear side of the prism, light absorbing means for absorbing light with the partial reflection of the stray light from the side of the prism being formed in part of the side of the prism.
In the invention as described in aspect 13, the light absorbing means is black paint.
An apparatus for watching around a vehicle as the invention described in aspect 14 comprises an imaging device including a casing disposed on the outer side of the vehicle and having a pair of right and left transmission window portions in either side of the casing, which casing contains a prism in the form of an isosceles triangle in section with its vertical angle directed to the front part of the casing, the right and left sides of the prism which correspond to the equal sides of the isosceles triangle being directed to the respective right and left transmission window portions, and an imaging element disposed to the rear of the prism and used for converting a ray of light into an image signal by concentrating light on an imaging plane via an imaging lens, the ray of light being incident on one of the right and left sides of the prism, reflected from the other side of the prism and emitted from the rear side of the prism, corner portions formed with the sides of the prism and the rear side of the prism being cut in a range excluding a range of horizontal viewing angles of the prism.
In the invention as described in aspect 15, a light scattering plane is formed on each of the surfaces formed by cutting the corner portions.
In the invention as described in aspect 16, black paint for absorbing stray light is applied to the light scattering plane.